1. Field of the Invention
The present invention relates generally to apparatus and methods for mechanically mixing and cooling a flow of particulate matter and more particularly to such apparatus and methods for mixing and cooling a packed bed flow of hot, retorted oil shale.
2. Background Discussion
Oil shales are sedimentary rocks containing solid, combustible organic material in a mineral matrix. This organic material, termed kerogen, is largely insoluble in petroleum solvents but, when heated in the absence of oxygen, decomposes to yield an oil similar to natural crude oil. Such oil shales are plentiful throughout the world and represent a vast, albeit a low grade, reserve of oil. According to recent estimates, oil shales capable of yielding between about 25 and 100 gallons of oil per ton (GPT) represent a worldwide oil reserve of about 9.times.10.sup.11 barrels (at 42 gallons per barrel). Considering all shales capable of yielding more than about 5 GPT, the worldwide shale oil reserve is estimated at about 5.5.times.10.sup.12 barrels. By comparison, the 1975 estimate of the world's reserve of natural crude oil was about 7.times.10.sup.11 barrels.
In addition to its vast oil reserve potential, shale oil, of all presently known alternative crude oils (generically referred to as "syncrudes"), most closely resembles natural crude oil. Using present technologies, a suitable feedstock for existing crude oil refineries can reportedly be produced from shale at a lower cost than from other syncrude sources.
According, for example, to the McGraw-Hill Encyclopedia of Energy, Second Edition, 1977 (pages 494-500), an estimated twenty percent of the United States' land mass overlies oil shale. The world's largest single shale oil reserve is believed to be the Eocene Green River Formation which covers about 16,500 square miles in Colorado, Utah and Wyoming. This particular formation is estimated to have an oil potential in excess of about 2.times.10.sup.12 barrels, of which about 6.times.10.sup.11 barrels are considered to reside in deposits having a potential of at least about 25 GPT. This latter reserve estimate for just the Green River Formation is about 20 times the recently estimated total natural crude oil reserves in this country.
Historically, in the late 1800's and early 1900's, substantial amounts of shale oil were reportedly produced in Europe. However, since then, when inexpensive crude oil started becoming readily available, it has almost always been the case that producing oil from shale has been more costly, and usually much more costly, than producing natural crude oil. Consequently, only in unusual circumstances, such as during World War II when the demand for petroleum products exceeded the amount of natural crude available and cost was not a controlling factor, has the production of oil from shale been known to be carried out on any substantial basis. After World War II, when cheap natural crude oil again became plentiful, interest in more costly shale oil decreased and remained low until the mid-east crises of the 1970's and the emergence of strong oil cartels caused a dramatic increase in the price of crude oil. In direct response thereto, and in the expectation that natural crude oil prices would continue to escalate, interest was renewed in all types of syncrudes, including shale oil. However, most, but not all, of this renewed interest in shale oil, along with other synfuels, lasted only until conservation practices and a general recession throughout the world, precipitated by the huge crude oil price increases of the mid-1970's and the resulting inflation, caused natural crude oil surpluses and a substantial fall in crude oil prices.
The commercially unattractive, high cost of producing shale oil, as compared with the cost of extracting natural crude oil from the ground or buying it abroad, is the result of many difficult technical and environmental problems, some of which are mentioned in the above-referenced McGraw Hill Encyclopedia of Energy at pages 18, 19 and 59. In turn, most of these problems relate, either directly or indirectly, to the large amounts of even high grade oil shale which must be extracted from the ground and processed to produce even moderate amounts of shale oil. As an example of the magnitude of this task, a moderate production rate of about 10,000 barrels of oil a day (BPD) from relatively high grade oil shale can be expected to require the mining and processing of between about 15,000 and 20,000 cubic yards (about 12,500 to 16,800 tons) of shale a day. Substantially greater amounts of lower grade shale must, of course, be extracted and processed to produce this amount of shale oil.
After being extracted from the ground, for example, by conventional room and pillar mining techniques, the large pieces of shale must generally be crushed into relatively small pieces (for example, into about two or three inch pieces) before the shale can be effectively retorted to convert the kerogen in the shale into a usable crude oil. However, shale crushing generally requires expensive crushing equipment because the high organic content of the shale makes the shale difficult to crush.
After being crushed to a relatively small size, the shale must then be retorted at a high temperature to extract the oil which must still, thereafter, be refined in the manner of natural crude oil. Retorting temperatures of several hundred degrees F. are typically required, but to obtain a high grade oil, which is low in olefins and saturates, retorting temperatures as high as about about 700.degree. F. to 1000.degree. F. are usually needed. Practical retort operation, to achieve a reasonable shale throughput rate, virtually dictates a continuous or substantially continuous feeding of shale into and through the retort, as opposed to batch processing. Very difficult problems are, however, typically associated with uniformly heating to a high temperature a large, continuous flow of shale through a retort in a manner which converts at least most of the kerogen in the shale to oil, it being obvious that the lower the yield, the more shale must be processed.
Because, at least employing present retorting processes, significant amounts of unconverted kerogen remain in the retorted shale and because a certain amount of shale coking inevitably occurs as a result of high temperature retorting, still other problems are commonly encountered with the safe and environmentally acceptable disposal of the hot retorted shale. In this regard, the amount of retorted shale to be disposed of amounts to about 80 weight percent of the amount of shale fed into the retort and the volume of the retorted shale is typically greater than that of the non-retorted shale.
If retorted shale is exposed to air at retort temperatures or, for that matter, at temperatures above about 500.degree. F., the kerogen and coke content can be expected to cause the shale to spontaneously ignite and start burning. Such burning of discharged, hot retorted shale makes the shale more difficult to dispose of and also may create environmental pollution problems. In some areas, such burning of retorted shale may be illegal. Consequently, retorted shale must ordinarily be cooled to a temperature of under about 500.degree. F. (assuming a retorting temperature higher than 500.degree. F.) before the shale can be safely and/or legally discharged and exposed to air.
In one known manner of cooling hot, retorted oil shale, the shale is discharged directly from the retort into a closed cooling system which comprises a cooling vessel or series of cooling vessels. Cooling water is sprayed onto the shale as it flows downwardly, under gravity, through the vessel or vessels. Presumably, by the time the shale flows through the vessel or vessels, it will have been sufficiently cooled to enable its discharge into the open. Exemplary apparatus for such shale cooling is disclosed in U.S. Pat. Nos. 4,556,458 to Deering et al. and 4,519,458 to Bertram.
It should be apparent that when high retorting temperatures are used, the shale requires substantial cooling. To achieve the amount of shale cooling needed under such circumstances, it is generally necessary to augment the cooling process by mixing the shale as it flows through the cooling vessel or vessels. Otherwise, the shale flow path may be required to be greater than can practically be provided because of space restrictions. Problems have, nevertheless, still been encountered with providing the amount of mixing needed to enable the adequate and efficient cooling of hot, retorted oil shale, particularly when the shale transit time and/or its travel path through the cooling vessels is limited by preexisting cooling system dimensional constraints.
Moreover, the wide range of retorted shale particle sizes, apparently caused by the shale being crushed and ground as it is fed through the retort, makes most retorted shale even more difficult to cool. This is because the small particles and fines fill otherwise open regions between larger shale pieces and block the flow of cooling water to these larger pieces. Furthermore, the fines and small particles tend to cling together and are difficult to wet.
In the absence of thorough and effective shale mixing in conjunction with water spray cooling, discrete regions or pockets of hot, essentially uncooled shale may exist and become entrained in the flow of retorted shale through the cooling vessel, as may regions or pockets of excessively wet shale. Whenever such pockets of uncooled shale and excessively wet shale encounter one another, the excess water may be explosively flashed into steam. The pressure surges caused by this steam flashing impedes both the flow of shale and the cooling process. Moreover, the pressure surges may feed back into the retort and disrupt the shale retorting process. Alternatively, or in addition, the pockets of uncooled shale can, upon discharge from the cooling system, spontaneously ignite, as described above, and regions of excessively wet shale can cause bridging in the cooling system, thereby further impeding the shale flow and/or cooling process.
An amount of shale mixing, insufficient to prevent the above-described problems associated with the formation of pockets of uncooled shale and excessively wet shale, usually cannot practically be compensated for by merely increasing the amount of cooling water used. The mechanisms causing the regions or pockets of uncooled shale are, for example, not significantly changed by adding more water, and the use of more water may increase, rather than decrease, the incidence of pressure surges caused by the mentioned flashing of water into steam and the amount of shale bridging. Furthermore, in many regions where oil shale may be mined and processed, the supply of water is limited and large amounts of water for shale cooling may not be available or may be prohibitively expensive. Even when water is available, the use of excessive amounts of water still adds to the cost of the shale cooling process, and thereby to the overall cost of shale oil production, when, in fact, shale oil production costs need to be reduced.
In spite of improvements which have been made to apparatus for mixing and cooling hot, retorted oil shale, additionally improved mixing and cooling apparatus are still sometimes required, especially for oil shale processing facilities in which the allocation of space or other constraints make the mixing and cooling of the retorted shale particularly difficult. It is, therefore, a principal objective of the present invention to provide such additionally improved apparatus which may be used to augment existing shale mixing and cooling apparatus and/or which may be used by itself for the effective mixing and cooling of hot, retorted oil shale or the like.